1. Field of the Invention
The present invention relates to record medium and apparatus useful in optical writing and/or reading of high density information and more particularly to improved configurations for optical discs and cooperating optical disc apparatus.
2. Description of the Prior Art
The currently preferred optical disc technology employs disc elements with spiral or concentric tracks of minute (e.g., on the order of a micron or less in size), optically-detectable marks. One real-time mode of recording (writing) such marks is by scanning tracks on the disc with an equivalently small beam of radiation (e.g., from a laser) which is modulated "off or on" according to an electrical signal representative of the information to be written. Information is recovered (read) by scanning the tracks with a larger, but still very tightly focused radiation (e.g. light) spot. The recovered information is in the form of a fluctuating electrical signal obtained from a photodetector that senses the read-out light reflected from the recorded disc. The benefits (e.g. high data storage density) inuring to recording with such small marks are very important; however, the problems encountered in precise implementation of the approach are very difficult. For example, focus of the write or read beam should be maintained with precision and the record surface should be protected e.g. from dirt and scratches.
More specifically, in order to write and read information in the form of such minute markings, optical systems of high numerical aperture are used to focus light to equivalently minute spots. Such optical systems have extremely small depths of focus and the proper positional relation between the writing or reading optical system and the optical disc record surface must be stringently maintained. Therefore it is highly desirable that the record layer, and thus optical disc support surface underlying the record layer, be smooth (relatively free of high-spatial-frequency variations from a nominal plane, e.g. such as caused by minute pits or bumps) and flat (relatively free of large amplitude, low spatial frequency variations e.g. such as caused by undulating surface variation of the support). Although complex focus-servo devices can effect lens adjustment (from a nominal focal zone) to compensate for imperfect smoothness and flatness, these devices add to the cost and fragileness of the write/read apparatus. The degree of complexity of the focus-servo devices is proportional to the degree of such disc imperfections and the speed of operation.
One approach to achieve requisite smoothness and flatness has been to form the disc substrate of glass with a ground and polished surface. This requires a time consuming and costly fabrication procedure. Another approach is to mold a plastic disc substrate with a highly finished surface and apply a surface smoothing sub-layer. However, it is extremely difficult to mold such plastic discs having adequate surface characteristics with a good yield; and this support fabrication method is also relatively costly.
Several different approaches have been suggested for protecting the disc recording layer surface from dirt and scratches. One is to overcoat that surface with a transparent protective layer of sufficient thickness to locate the exterior protective surface out of the depth of focus of the optical system. However, protective layers of this thickness require complicated optical correction of aberrations and can present problems in certain deformation recording processes for the recording layer. An alternative approach described in U.S. Pat. No. 4,038,663 is to centrally attach a flexible disc cover (in spaced relation to the record surface of a rigid optical disc) and "fly" the cover sheet above the record layer during write or read operations. Another approach disclosed in U.S. Pat. No. 4,074,282 provides inner and outer coaxial spacers to support a transparent cover plate in spaced relation from the record surface of a rigid optical disc.
While the above-noted and other protective layer or protective cover member approaches are useful to avoid scratch or dirt problems, they do not simplify the above-noted problems in meeting precise smoothness and flatness requirements.